An artificial knee joint, or knee prosthesis, is used to replace a biological knee if the latter is subjected for example to primary or secondary arthrosis, due to a trauma or caused by infections, post-traumatic arthrosis, rheumatoid arthritis, inflammatory arthritis, meniscectomy, osteo-necrosis, or bone tumors, or again if the biological knee is subjected to particularly serious traumas or other similar problems.
Known knee prostheses typically comprise a femoral component, which is attached to the distal end of the femur, and a tibial component, which is attached to the proximal end of the tibia.
The femoral component, as its main components, normally has a medial condyle, a lateral condyle, partly separated from each other by an intercondyle groove, and a front flange.
The tibial component normally comprises a tibial plate, which is attached during use to the proximal end of the tibia, and an insert, which comprises a medial articulation surface and a lateral articulation surface, on which respectively the medial condyle and the lateral condyle of the femoral component articulate during use.
The condyles and the articulation surfaces are all in all configured to reproduce a movement similar to that of a healthy biological knee.
More specifically, the femoral component and the tibial component are normally configured to reduce as much as possible the probability that incorrect movements occur due to anomalous translations in the front, rear, medial or lateral directions, and rotations.
In surgical operations that use a total knee prosthetic implant, the anterior cruciate ligament is generally removed.
Instead, the posterior cruciate ligament is removed if its functionality is deemed compromised or if the functionality is totally non-existent, for example due to a pathological evolution.
The presence of the two collateral ligaments, that is, the medial and lateral ligaments, is necessary for the overall stability of the artificial joint.
When the anterior and posterior cruciate ligaments have been removed and at the same time a good functionality of the collateral ligaments is maintained, it is necessary to use a prosthetic configuration that allows to restore the original mechanical stability.
This configuration commonly provides, in some known prosthesis solutions, a tibial post, or simply post, with a proximal prominence, provided on the tibial insert in its central zone, that is, positioned between the medial and lateral articulation surfaces of the tibial insert itself.
The tibial post is inserted inside the intercondyle groove of the femoral component when the prosthesis is implanted.
In some known prosthesis solutions, the post also interferes with a femoral cam, or simply cam, located in proximity to the rear end of the intercondyle groove, or in a position opposite the front flange, to restore the functionality of the posterior cruciate ligament that has been removed.
The post and the cam usually articulate for a reduced portion of the flexion of the joint.
During the flexion of the leg, a rotation movement also occurs of the biological knee with respect to the tibial axis toward the outside, and this rotation also occurs in a prosthesized knee provided with an artificial joint.
The amplitude of such rotations depends in particular on specific conditions of the individual patient. Therefore, it is not recommended to facilitate the external rotation through contact between post and cam for a fixed degree of rotation, but it is advantageous to provide an optimized geometry for post and cam for a wider range of external rotation.
Therefore, for this reason, the areas of post and cam intended to enter into reciprocal contact during rotation are designed not congruently, through an asymmetrical design, so as to allow a sufficiently wide range of variability and to supply contact for different degrees of external rotation.
The prior art documents EP 1 591 082 B1, US 2012/0143342, US 2007135925, U.S. Pat. No. 6,013,103, EP 0941719 A, U.S. Pat. No. 5,549,686, and the scientific article by Cates et al. “In Vivo Comparison of Knee Kinematics for Subjects Having Either a Posterior Stabilized or Cruciate Retaining High-Flexion Total Knee Arthoplasty”, describe solutions for articular knee prostheses that are proposed at least partly to solve the problems deriving from external rotation during flexion.
The solutions proposed in these prior art documents are not completely satisfactory however, because in some forms of embodiment the cam has a symmetrical profile that does not allow a suitable same-shape coupling of the cam and post during external rotation.
Another disadvantage of some known forms of embodiment is that the external rotation is forced by the contact between the femoral cam and the tibial post during the flexion movement, which makes this movement less natural and at the same time leads to an increase in wear, of both the cam and the post.
The increase in wear can also lead to a reduction in the life of the knee prosthesis.
In some known forms of embodiment, the tibial insert has an asymmetrical contact surface of the post, intended to enter into contact with the cam.
This asymmetry does not allow to exchange the right and left femoral component using a single tibial insert.
It is therefore a purpose of the present invention to obtain an artificial knee joint that facilitates the natural movements of the leg, completely reproducing the natural kinematics of a healthy knee.
Another purpose of the present invention is to obtain an artificial knee joint that reduces the stresses deriving from the interaction between femoral component and tibial component.
The Applicant has devised, tested and embodied the present invention to overcome the shortcomings of the state of the articulation and to obtain these and other purposes and advantages.